FMF - Friends of Minerals Forum, discussion and message board

[fuss]

The rock is a grey calcite with some minor willemite and what I thought was franklinite labeled only "Franklinite NJ" I picked up from a local rock shop.
When I put a magnet to it it snaps to it like other magnetite samples I have vs the weaker attraction of my other franklinite samples, so I'm wondering if these could be magnetite crystals instead?

[fuss]

another shot.

[alfredo]

I can‘t answer your question, Fuss, but I would like to comment that there is no sharp boundary visually or in physical properties between franklinite and magnetite as they are part of a compositional series. Iron and manganese can substitute for the zinc in franklinite, so the degree of magnetism can vary. The distinction between magnetite and franklinite depends on whether ferrous iron or zinc dominates in the divalent metals position in the structure, and both metals are likely present. You would need a chemical analysis to be certain which one was dominant although, as you pointed out, the strong magnetism does increase the likelihood that it is magnetite.

[fuss]

Thank you Alfredo, that makes sense, one has zinc the other not.

[DoveOfLight]

Alfredo, I agree with you and wanted to add a clue is probably in the host rock, I would like to call it spinel supergroup.

[cascaillou]

File down a piece of iron to get some very fine iron dust, and see if this dust is attracted by the tiny crystals and sticks to it.